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During descent of a spacecraft to any planetary body the spacecraft uses retro engine with variable throttle to reduce the vehicle velocity to zero at touchdown.

Inorder to compute for the amount of throttle required the guidance needs mass and inertia(for RCS) of the spacecraft.

However, the mass and inertia are changing constantly as fuel is being used. It is definitely possible to compute onboard how much fuel is remaining based on throttle history. But this will result in error accumulation over time and probably the margins of closed loop controller will be exhausted.

So my question is... do spacecrafts employ some fuel measuring device on board? Like the way cars do ! However same device principle will obviously not work.

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In order to compute for the amount of throttle required the guidance needs mass and inertia (for RCS) of the spacecraft.

What's actually important is to know your position, velocity, and acceleration. Estimates of mass are obviously useful but don't have to be 100% accurate if you have closed-loop control and good knowledge of your current acceleration.

For position and velocity, you'll want something like a radar altimeter and/or GPS.

An on-board accelerometer takes care of any errors in either the mass or thrust estimate, and automatically incorporates any atmospheric effects if you're landing on a body with an atmosphere.

Direct measurement of propellant level would be subject to errors due to sloshing; it's possible that dead-reckoning estimates of prop levels based on flow rate would be more accurate.

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Measure of propellant level in zero-gee tankage is a tricky problem.

Shuttle attempted it in its Orbital Manuevering System.

A capacitance gauging system in each OMS propellant tank measures the propellant in the tank. The system consists of a forward and aft probe and a totalizer. The forward and aft fuel probes use fuel (which is a conductor) as one plate of the capacitor and a glass tube that is metallized on the inside as the other. The forward and aft oxidizer probes use two concentric nickel tubes as the capacitor plates and oxidizer as the dielectric. (Helium is also a dielectric, but has a different dielectric constant than the oxidizer.) The aft probes in each tank contain a resistive temperature-sensing element to correct variations in fluid density. The fluid in the area of the communication screens cannot be measured.

The system was problematic and Orbiters often flew with it nonfunctional.

enter image description here

The Orbiter's Reaction Control System tanks used algorithmic gauging

The RCS quantity monitor uses the GPC to calculate the usable percent of fuel and oxidizer in each RCS module. The quantities are computed based on the pressure, volume, and temperature method, which requires that pressure and temperature measurements be combined with a unique set of constants to calculate the percent remaining in each of the six propellant tanks. Correction factors are included for residual tank propellant at depletion, gauging inaccuracy, and trapped line propellant. The computed quantity represents the usable (rather than total) quantity for each module and makes it possible to determine if the difference between each pair of tanks exceeds a preset tolerance (leak detection). The sequence assumes that helium flows to the propellant tanks to replace propellant leaving. As a result, the computed quantity remaining in a propellant tank will be decreased by normal usage, propellant leaks, or helium leaks.

(GPC = General Purpose Computer)

Propellant transferred between the two systems was estimated by a burn-time algorithm.

Reference 1

Just for completeness, the shuttle's External Tank didn't have a sophisticated gauging system for its propellant. Instead, simple on/off level sensors were used to aid in filling the tank, and to shut off the engines if the tank ran dry prematurely.

enter image description here

Reference 2

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  • $\begingroup$ Thanks ! Exactly what I was looking for. But it seems interplanetary landers dont employ any such technique. $\endgroup$ – Prakhar Apr 24 '18 at 1:00
  • $\begingroup$ It's possible the used some variant of the algorithmic gauging approach. The tank-probe approach didn't work very well. $\endgroup$ – Organic Marble Apr 24 '18 at 1:27

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